While the United States doesn't have a national religion, the US has fairly deep religious liberty roots, and it has lead to a few twists and turns in the religious sentiment
Every year, Americans celebrate Thanksgiving (a major US holiday), and most children could tell you some version of the story of the Pilgrims and Native Americans. The truth is, it was a religious holiday long before Abraham Lincoln formalized it
Pilgrims and Puritans who emigrated from England in the 1620s and 1630s carried the tradition of Days of Fasting and Days of Thanksgiving with them to New England. The modern Thanksgiving holiday tradition is traced to a well-recorded 1619 event in Virginia and a sparsely documented 1621 celebration at Plymouth in present-day Massachusetts. The 1619 arrival of 38 English settlers at Berkeley Hundred in Charles City County, Virginia, concluded with a religious celebration as dictated by the group's charter from the London Company, which specifically required "that the day of our ships arrival at the place assigned ... in the land of Virginia shall be yearly and perpetually kept holy as a day of thanksgiving to Almighty God." The 1621 Plymouth feast and thanksgiving was prompted by a good harvest, which the Pilgrims celebrated with Native Americans, who helped them get through the previous winter by giving them food in that time of scarcity.
Then there were the religious immigrants of the 19th and 20th centuries. Jews, for instance, migrated from an openly hostile Russia (a story popularized in a 1986 children's movie). There are many Christian immigrants with similar stories. People still immigrate to the US for religious reasons (Christian or not) to this day.
The 1950's "War" with Communism
After World War II, the Cold War brought proxy wars between the US and Soviet Russia. One point of tension was that the United States was seen as a "Christian nation", while the Soviets were staunch Atheists. It was well known there was active hostility of the Soviets against religion
Orthodox churches were stripped of their valuables in 1922 at the instigation of Lenin and Trotsky. In subsequent years, including both the Stalin and the Khrushchev periods, tens of thousands of churches were torn down or desecrated, leaving behind a disfigured wasteland that bore no resemblance to Russia such as it had stood for centuries. Entire districts and cities of half a million inhabitants were left without a single church. Our people were condemned to live in this dark and mute wilderness for decades, groping their way to God and keeping to this course by trial and error. The grip of oppression that we have lived under, and continue to live under, has been so great that religion, instead of leading to a free blossoming of the spirit, has been manifested in asserting the faith on the brink of destruction, or else on the seductive frontiers of Marxist rhetoric, where so many souls have come to grief.
This let McCarthyism (where Communists in America were hunted down by McCarthy) define atheism as de-facto Communist sentiment
Today we are engaged in a final, all-out battle between communistic atheism and Christianity. The modern champions of communism have selected this as the time. And, ladies and gentlemen, the chips are down—they are truly down.
As such, the 1950s saw a rapid rise in religious participation
Churches and schools were being greatly expanded to accommodate the growing population, and organized religion was in its heyday. On a typical Sunday morning in the period from 1955-58, almost half of all Americans were attending church – the highest percentage in U.S. history. During the 1950s, nationwide church membership grew at a faster rate than the population, from 57 percent of the U.S. population in 1950 to 63.3 percent in 1960.
Much of this generation (the "baby boomers") are still alive and have passed on their faith to their children. Consider that the Democrat's "liberal lion" Ted Kennedy was responsible for pushing the Federal Religious Freedom Restoration Act in the Senate, which drew bipartisan support.
The Moral Majority
In 1979, Jerry Falwell Sr. founded The Moral Majority, which sought to mix Christianity with political conservatism and eventually the Republican party. It helped to elect Ronald Reagan and George H.W. Bush and defined a period of time in America where Conservative Christianity was a political force to be reckoned with.
It's shortly after this we start to see some significant generational declines in religious participation
Why has America leveled off?
Despite the intermingling of politics and religion, America still has a large network of churches in most communities, and these churches are not going quietly into the night. Southern Baptists (the largest protestant denomination in America) has a fairly robust church planting and outreach ministry. The Catholic Church runs a large network of hospitals and they also have a fairly large charitable organization. With a stable base of legacy Christians still supporting them, America's churches continue to impact their local communities, which helps drive membership.